In spite of decades of research on stuttering, there are few unequivocal findings. Several reasons for this are offered and discussed. Particular attention is paid to the lack of one accepted definition of stuttering. Other issues concern the unit of stuttering, inter-and intra-stutterer variability, the overt and covert features if stuttering, objective and subjective measures in stuttering research and the sampling of material for study. These are some of the problems which pertain to study

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... the nature of stuttering. They apply also to research on therapy, which presents additional challenges to the researcher.